1. Field of the Invention
This invention relates to a voltage sensor used for sensing a voltage on an electrically charged object such as a distribution line.
2. Description of the Related Art
In a conventional zero-phase-sequence voltage sensing apparatus, potential transformers (hereinafter referred to as the "PTs") or capacitance potential devices (hereinafter referred to as the "PDs") are used as voltage sensors installed associated with different phase distribution lines. In addition, voltage sensors, using optoelectronic techniques, other than PTs also have been recently proposed. The voltage sensors using optoelectronic techniques have been called optical potential transformers (hereinafter referred to as the "OPTs"). There are many such OPTs studied which utilize the fact that when a linear polarization enters an element having Pockels effect, put under an electric field, crossing at a right angle with the electric field, the refractive indexes for two orthogonal components of the polarization exhibit different changes and also propagation velocity varies depending upon the intensity of the filed and as a result the two optical orthogonal components differ in phase, so that the output light becomes elliptically polarized.
Conventional PTs or PDs such as those mentioned above are of the contact type in which they are fixed directly to electrically charged objects such as cables included in the distribution lines. Thus electrical isolation must be considered. In addition, there is the problem that since PTs or PDs use a coil, a core, capacitors, etc., the entire arrangement will be large and heavy and thus the time required for fixing them will increase.
Since OPTs are disposed in noncontact relationship to the distribution lines, they are good in terms of isolation. They, however, need a laser oscillator, etc., in order to obtain a linear polarization and thus the entire arrangement becomes expensive.